Globally, invasive rats are one of the major problems affecting seabirds breeding on islands.
When rats jump ship and colonise, their predation of eggs and chicks can lead to massive slumps in seabird numbers.
In turn, this makes the rain of seabird poo washing out to surrounding waters shrink from a torrent to a trickle.
Surprisingly, for jewel damselfish living on coral reefs there, that affects their habits – including how they react to other fish.
Jewel damselfish are aggressively territorial and will attack intruders – even much larger fish – that stray into territories they hold over small patches of reef. The fish are also capable of ‘farming’ turf algae that grow on coral branches.
This plant material is their staple food, but not all such algae are equal.
Around islands with healthy seabird populations, the food value is higher, thanks to nutrient boosts from guano.
That allows jewel damselfish to hold smaller territories than on reefs around islands where rats are present and fertiliser from seabirds reduced.
Now further links between land and sea have been revealed through research reported in Nature Ecology and Evolution by behavioural ecologist Rachel Gunn and co-workers.
This shows that jewel damselfish beside islands where black rats live are also less belligerent in defending their territories.
“Jewel damselfish around rat-free islands aggressively defend their turf because the higher enriched nutrient content means they get ‘more for their money’, and this makes it worth the energy cost needed to defend,” says Gunn.
By contrast, the lower food value of turf algae around rat-infested islands makes it barely worth the effort of territorial defence.
“The algal farming of damselfish affects the balance of corals and algae on the reef. Their aggression towards other fish can influence the way those fish move around and use the reef,” she adds.
“We don’t yet know what the consequences of this behavioural change will be, but ecosystems evolve a delicate balance over long time-scales, so any disruption could have knock-on consequences for the wider ecosystem.”
Main image: Researchers from the Universities of Lancaster and Tübingen have found that invasive rats alter jewel damselfish behaviour. © Waterframe/Getty